The MT3000 is well supported by the official OpenWrt project. Feel free to install it from here:
That said, if you continue to use the GL-inet firmware, you'll need to ask in their support channels since their firmware is highly customized for their products.
Yes, I can confirm, I own the CN variant of MT3000, just flash to vanilla OpenWrt and it's basically the same as the global model, that country setting is only affecting GL-INET's firmware UI